a) Field of the Invention
The present invention relates to video, voice, and data communications in a CATV (cable television) network. More particularly, the present invention is capable of improving return path signal quality of the traditional HFC (hybrid fiber coax) network architectures used by CATV service providers, consequently allowing for longer transmission distances, higher signal modulation formats in the order of higher order of M-QAMs, and higher network reliability.
b) Description of the Prior Art
Communications networks today are required to provide higher and higher bandwidth as subscriber bandwidth demands grow due especially to Internet applications such as file sharing, video conferencing, ecommerce, consumer video production and subsequent posting to sites such as Youtube, etc. While many ILECs (incumbent local exchange carriers) and CLECs (competitive local exchange carriers) have taken the approach of completely rebuilding their networks using fiber-based technologies such as GPON, this has resulted in both expensive plant upgrades and equipment replacement. Existing copper plants are replaced by fiber builds. DSL modems are replaced with GPON ONTs and IP settop boxes are deployed to consumer homes, while complete digital headends have to be built.
CATV operators have in the past 20 years progressed from being entertainment providers to being also broadband providers and then voice service providers. However, CATV operators face the same bandwidth demand growth. Different solutions have been applied, which include higher modulation schemes in both forward path (downstream) and return path (upstream) signals. Currently many CATV operators use 256 QAM modulation for downstream signals, and are actively migrating from 16 QAM to 64 QAM for upstream return path signals.
As higher levels of modulation are used, the required signal to noise ratio (signal quality) increases. This means that the good signals originating from cable modems, eMTA and cable settop boxes must be at a sufficient power level above unwanted interfering noise to ensure good data transmission quality. Furthermore, the popular use of eMTAs for VoIP service means that data communications over the HFC network need to progress from previous a best-effort service to a guaranteed level to ensure quality of voice communications.
In an HFC plant, most of the unwanted noise signals enter the HFC plant from the home. This can be caused by any combination of unterminated coaxial F-ports; bad shielding of televisions, VCRs, or cable boxes; and low quality RF amplifiers with either bad shielding, self oscillations, return loss, or distortions which all combine to allow ingress of noise.
HFC networks employ the DOCSIS standard for bi-directional data transmission. The DOCSIS cable modem and eMTA in the home transmits return path data as needed in bursts. This means that when not actively transmitting data, the cable modem is inactive. Cable settop boxes also use a burst mode transmission pattern, sending return path signals only when the home user orders a particular movie from the VOD or PPV service. However, despite of the burst nature of return path signals, present HFC plant design provides for a return signal path that is always open, so that ingress noise is transmitted even though no active transmission is taking place.
Since ingress noise from all the homes are added together because of the noise funneling effect of the HFC plant (FIG. 1), this noise funneling effectively sets the limit on the number of homes per node, as well as the highest modulation level that can be used. In order to reduce ingress noise so that higher modulation levels can be used, CATV operators have to reduce node sizes, which require expensive HFC plant upgrades consisting of new optical fiber deployment and capital equipment investment.